JP6364470B2 - Electromagnetic flowmeter with automatic adjustment function based on detected complex impedance - Google Patents

Description

  The present invention relates to fluid processing, and more particularly to measurement and control of process fluids. Specifically, the present invention relates to a measurement technique for an electromagnetic flow meter.

  The electromagnetic flow meter measures the flow rate by Faraday's law of electromagnetic induction, that is, electromagnetic action. In this flow meter, a coil is driven to generate a magnetic field across a pipe line, and an electromotive force (EMF) across the flow of the process fluid is induced by the magnetic field. The potential difference (i.e., voltage) thus generated is measured by a pair of electrodes extending through the pipe wall and in contact with the process fluid, or by electrostatic coupling. The flow rate is proportional to the induced EMF, and the volume flow rate is proportional to the flow rate and the channel cross-sectional area.

  In general, the electromagnetic flow measurement technique can be applied to the flow of a fluid containing water as a main component, an ionic solution, and other conductors. Specific applications include water treatment facilities, high-purity chemical production, sanitary food and beverage production, and chemical treatment including hazardous and corrosive process fluids. Electromagnetic flow meters are also used in the hydrocarbon fuel industry and other hydrocarbon extraction processes that involve hydraulic fracturing techniques using abrasive and corrosive slurries.

  Electromagnetic flowmeters provide quick and accurate flow measurement in applications where differential pressure technology is not preferred due to the accompanying pressure loss (eg, pressure loss when passing through an orifice plate or venturi). I will provide a. An electromagnetic flow meter can also be used when it is difficult or impossible to introduce mechanical elements such as a turbine rotor, vortex generating member, or pitot tube into the process fluid.

  Some types of electromagnetic flow meters use field coils that are driven directly by AC power. In another type of electromagnetic flow meter, generally called a pulsed DC excitation electromagnetic flow meter, the field coil is periodically driven by a low-frequency rectangular wave. This pulsed DC excitation type electromagnetic flow meter uses a magnetic field whose direction is switched at a specific frequency. When the magnetic field is reversed, a spike occurs in the electrode voltage due to the rapid reversal of the magnetic field. Such spikes are independent of flow velocity, but are related to the rate of change of the magnetic field. In order to measure the flow rate, the electrode voltage measurement circuit must be programmed to wait until this spike is completely attenuated, otherwise it was caused by the decaying spike. The potential difference will appear in the flow measurement.

  The rate of decay of the voltage spike varies with the impedance of the process fluid acting as a resistive capacitance (RC) filter on the voltage spike. On the other hand, this impedance depends on the conductivity of the process fluid, some coating on the electrode surface, and the length of the cable between the transmitter and the channel tube. Therefore, when the flow meter is shipped from the manufacturing plant, it is not possible to know the rate of decay of the voltage spike, and if the conductivity of the process fluid changes or a coating is formed on the electrode surface, the rate of decay of the voltage spike May change over time.

  Since the impedance between the electrodes cannot be known in advance and may change over time, the pulsed DC excitation type electromagnetic flow meter may cause voltage spikes to enter the flow measurement value. It is common to program to sample only at the very last part of a half cycle, which is very low. Such a measurement period may be, for example, only 20% of each half cycle.

The present invention relates to an electromagnetic flow meter that automatically adjusts operating parameters based on detected complex impedance. The electromagnetic flow meter includes a pipe, a coil adjacent to the outer peripheral surface of the pipe, a current supply source, a plurality of electrodes extending to the inner peripheral surface of the pipe, and the plurality of electrodes. And a processor for generating a flow rate output signal corresponding to the detected voltage between the two. The processor has a coil driving frequency according to a complex impedance detected between the plurality of electrodes or between one or more of the plurality of electrodes and a ground as a complex impedance of a process fluid flowing in the pipeline. Adjusting one of the electrode voltage sampling period and the phase shift of the electrode signal .

A further aspect of the present invention relates to a flow rate measuring method. The flow measurement method includes a step of generating a magnetic field across the flow of a process fluid, a step of detecting a voltage induced between a plurality of detection electrodes by the magnetic field in an electrode voltage sampling period, and the detected voltage And generating a flow rate output signal. Further, the flow measurement method detects a complex impedance of the process fluid as a complex impedance between the plurality of detection electrodes or between one or more of the plurality of detection electrodes and a ground. And changing one of a coil driving frequency, an electrode voltage sampling period, and an electrode signal phase shift according to the complex impedance.

In another aspect, the method for operating an electromagnetic flowmeter detects a complex impedance between a plurality of detection electrodes or between one or more of the plurality of detection electrodes and a ground as a complex impedance of a process fluid. And automatically adjusting one of a coil drive frequency, an electrode voltage sampling period, and an electrode signal phase shift of the electromagnetic flow meter according to the detected complex impedance. .

It is a block diagram of a pulsed direct current excitation type electromagnetic flow meter. It is a block diagram which illustrates embodiment which adjusts the electrode voltage sampling period which is an operation parameter according to the detected complex impedance. It is a block diagram which illustrates embodiment which adjusts the coil drive pulse frequency which is an operation parameter according to the detected complex impedance. It is a graph which shows an electrode signal according to progress of time regarding the pulsed direct current | flow excitation type electromagnetic flowmeter at the time of satisfy | filling a pipe line with the water which has two different electrical conductivity, respectively. The present invention relates to a pulsed direct current excitation type electromagnetic flow meter when water is filled with water having two different conductivities, and an electrode signal is shown over time and an electrode based on a detected complex impedance. It is a graph which illustrates automatic adjustment of a voltage sampling period.

  FIG. 1 shows a typical pulsed direct current excitation type electromagnetic flow meter 10, which is composed of a main part (that is, a flow channel) 10 </ b> A, an attachment part (that is, a transmitter) 10 </ b> B, and the like. Is provided. 10 A of flow-path pipe parts are provided with the pipe line 12, the liner 14 for insulation, the electrode 16A, the electrode 16B, the field coil 18A, and the field coil 18B.

  The main function of the flow channel section 10A is to generate a voltage proportional to the flow velocity of the fluid to be measured. The field coil 18A and the field coil 18B are driven by a current flowing to generate a magnetic field. In the pulsed direct current excitation type electromagnetic flow meter, the direction of the magnetic field generated by the field coils 18A and 18B is switched by periodically reversing the direction of the coil drive current. The process fluid that passes through the interior of the flow path tube portion 10A functions as a moving conductor that induces a voltage in the fluid. The two electrodes 16A and 16B attached so as to be flush with the inner peripheral surface of the flow path tube portion 10A are generated in the process fluid by being in direct electrical contact with the conductive process fluid. Capture voltage. In order to prevent this voltage from shorting out, the process fluid must be contained in an electrically insulating material. When the conduit 12 is a metal tube, this electrical insulation is performed by a liner 14, which is made of a non-conductive material such as polyurethane, polytetrafluoroethylene (PTFE), or an insulating rubber material.

  The transmitter 10B analyzes the voltages generated at the electrodes 16A and 16B and transmits a standardized signal to a monitoring system or a control system. The attachment portion 10B is generally called a transmitter or a signal converter.

  Usually, the transmitter 10B includes a signal processor 20, a digital processor 22, a coil driving unit 24, and a communication interface 26. Signal conversion, adjustment and transmission are the main functions of the transmitter 10B.

  The digital processor 22 controls the pulse frequency of the pulsed DC coil drive current supplied to the two field coils 18A and 18B by the coil drive unit 24. The waveform of the current supplied by the coil drive unit 24 is a rectangular wave having a frequency indicated as a pulse frequency. The coil drive is periodically interrupted and the two field coils 18A and 18B are deactivated. Thereby, the voltage between the electrode 16A and the electrode 16B or between the electrode 16A or the electrode 16B and the ground can be periodically measured. Such measurement can be used for sampling of the voltage induced in the flow path tube portion 10A and correction for noise. This measurement can also be used to detect complex impedance, which automatically adjusts the operating parameters of the electromagnetic flow meter 10 such as the electrode voltage sampling period and coil drive pulse frequency. Can be used when

  The complex impedance can be detected without stopping the operation of the field coil. The complex impedance is detected by flowing current through the two field coils 18A and 18B at least one of a frequency that is away from the coil driving frequency and a frequency that is asynchronous with the drive signals of the two field coils 18A and 18B. While doing so, it is possible to continue measuring the flow rate.

  The signal processor 20 is connected to the electrodes 16A and 16B and a ground point. The connection to the ground point may be a connection to the pipe line 12 or a connection to a flange or a pipe part on the upstream side or the downstream side of the pipe line 12.

  Over the electrode voltage sampling period defined by the digital processor 22, the signal processor 20 monitors the potential VA of the electrode 16A and the potential VB of the electrode 16B. The signal processor 20 generates a voltage representing the potential difference between the electrode 16A and the electrode 16B, and converts this voltage into a digital signal indicating the electrode voltage during the electrode voltage sampling period. The digital processor 22 may further perform signal processing and maintenance of the digital signal received from the signal processor 20. The digital processor 22 sends a flow rate measurement value to the communication interface 26, and the communication interface 26 performs communication for reading the flow rate measurement value and transmitting it to a control system (not shown). The communication by the communication interface 26 is an analog current value that changes between 4 mA and 20 mA, and a HART (registered trademark) protocol in which digital information is modulated to a current of 4 to 20 mA, such as Fieldbus (registered trademark, IEC 61158). It is possible to use a communication protocol via a simple digital bus or wireless communication via a wireless network using a wireless protocol such as WirelessHART (registered trademark, IEC62591).

  The signal processor 20 can also monitor the complex impedance in the flow path tube portion 10A. The complex impedance is determined based on the conductivity of the fluid flowing in the flow path tube portion 10A, the electrodes 16A and 16B. It varies depending on any coating formed on the surface and the length of the cable between the flow path tube 10A and the transmitter 10B. In this detection function, the signal processor 20 applies a current to the electrode 16A and the electrode 16B. The voltage between the electrode 16A and the electrode 16B, or the voltage between one of the electrode 16A and the electrode 16B and the ground can be used to obtain a detection value of the complex impedance by the digital processor 22, and this complex The detected impedance value can be used for automatic adjustment of the operating parameter of the electromagnetic flow meter 10 as shown in FIGS. 2A and 2B. FIG. 2A is a block diagram illustrating an embodiment in which the electrode voltage sampling period of the operating parameter is adjusted according to the detected complex impedance, and FIG. 2B is a diagram illustrating the coil driving pulse frequency of the operating parameter according to the detected complex impedance. It is a block diagram which illustrates embodiment to adjust. As shown in the embodiment of FIG. 2A, complex impedance is detected (step 100), and then an electrode voltage sampling period is determined according to the detected complex impedance (step 102). Finally, the actual electrode voltage sampling period is adjusted to an electrode voltage sampling period determined according to the detected complex impedance to improve the signal-to-noise ratio (step 104). Similarly, in the case of the embodiment of FIG. 2B, complex impedance is detected (step 200), and then the coil drive pulse frequency is determined according to the detected complex impedance (step 202). Finally, the actual coil drive pulse frequency is adjusted to a coil drive pulse frequency determined according to the detected complex impedance, thereby improving the signal-to-noise ratio (step 204). In one embodiment, the detected complex impedance may be used to adjust both the electrode voltage sampling period and the coil drive pulse frequency.

  Thus, based on the detected complex impedance, the digital processor 22 can change one or both of the electrode voltage sampling period and the coil drive pulse frequency to improve the signal to noise ratio. Specifically, the electrode voltage sampling period may be adjusted in the opposite direction to the detected change in complex impedance, and the electrode voltage sampling period may be increased as the detected complex impedance decreases. Further, the coil drive pulse frequency may be adjusted in the direction opposite to the change of the complex impedance, and the coil drive pulse frequency may be increased as the detected complex impedance decreases.

  FIG. 3 is a graph showing electrode signals as time elapses with respect to a pulsed DC excitation type electromagnetic flow meter. This graph was obtained from a Rosemount® 8705 flow meter when each 6 inch (152.4 mm) line was filled with water having two different electrical conductivities. . The coil drive pulse frequency is set to 37 Hz. The first sample is deionized water (DW) having a conductivity adjusted to 5 μS / cm, which is the minimum conductivity defined in the Rosemount 8705 flow meter. The second sample is standard tap water (TW) with conductivity measured at 220 μS / cm.

  As shown in region A of FIG. 3, the amplitude of the spike generated by reversing the direction of the magnetic field is smaller in the deionized water (DW) waveform than in the tap water (TW) waveform.

  Region B shows that the falling edge of the spike in the DW waveform is delayed from the falling edge of the spike in the TW waveform. Region C shows that spikes in the DW waveform decay more slowly than spikes in the TW waveform.

  FIG. 4 also shows the waveform of deionized water (DW) and the waveform of tap water (TW). The coil drive pulse frequency is 37 Hz. Deionized water has a conductivity of 5 μS / cm and tap water has a conductivity of 220 μS / cm.

  In FIG. 4, a 15-cycle moving average having a DWRA and TWRA code is superimposed on a TW signal and a DW signal. These undulating average waveforms make it easy to see the spike decay over the noise.

  The graph shown in FIG. 4 shows a standard 20% electrode voltage sampling period used in a pulsed DC excitation type electromagnetic flow meter. This standard 20% electrode voltage sampling period is labeled STD in FIG.

  Further, as indicated by a broken line in FIG. 4, an increase in the electrode voltage sampling period is indicated by a symbol ADD. This shows an additional sampling period applicable for fluids with high conductivity.

  In the case of a sample of deionized water, the spike does not decay near zero until just before the standard 20% electrode voltage sampling time STD begins. Therefore, the electrode voltage sampling period for deionized water is generally optimal.

  On the other hand, in the case of the tap water sample, the spike is completely attenuated well before the start of the standard 20% electrode voltage sampling time STD. This means that a longer sampling period can be applied, and as a result, a higher signal-to-noise ratio can be obtained. In the example shown in FIG. 4, the electrode voltage sampling period STD + ADD can be made approximately three times the standard 20% electrode voltage sampling period STD.

  Whether a longer electrode voltage sampling period can be employed depends on whether it is possible to measure the complex impedance of the process fluid that allows estimation of the decay time. Many process fluids that flow through an electromagnetic flow meter have a conductivity that is equivalent to or greater than that of tap water, and in many cases of flow measurement using an electromagnetic flow meter, the electrode voltage sampling period The effect of extension will be obtained.

  The performance of the electromagnetic flow meter can be improved by increasing the signal-to-noise ratio. Alternatively, power supplied to the field coil may be reduced without impairing performance. A fluid having a greater conductivity than tap water can be stabilized more quickly, resulting in a greater improvement.

  The effect of the automatic adjustment of the electrode voltage sampling period according to the detected complex impedance can be obtained without causing a significant increase in cost. Typically, changes to the electronics in the transmitter are minimal, and complex impedance detection and electrode voltage sampling periods can be made by software changes in the digital processor 22.

  As another embodiment of the present invention, the coil drive pulse frequency can be automatically adjusted based on the complex impedance of the fluid. In general, the higher the frequency, the lower the process noise. Therefore, the signal-to-noise ratio is improved by increasing the coil drive pulse frequency. However, voltage spikes are not completely attenuated, and the decay time cannot be determined without considering the complex impedance of the fluid. There is. By detecting the complex impedance of the fluid and adjusting the coil drive pulse frequency according to the complex impedance, the transmitter 10B allows the electromagnetic flow meter 10 to be operated at the highest possible frequency without impairing the stability of the zero point. It can be activated.

  The adjustment of the operation parameter of the electromagnetic flow meter based on the detected complex impedance is also applicable to an AC excitation type flow meter. For example, the detected complex impedance may be used to adjust the phase shift of the electrode signal based on the electrode impedance. As a result, the better quadrature phase adjustment can be obtained, so that zero drift can be suppressed, and noise reduction according to the measurement technique used can be further improved.

  Although the present invention has been described based on specific embodiments, various modifications can be made without departing from the scope of the present invention, and each component of the present invention can be replaced with equivalents. Will be understood by those skilled in the art. In addition, various modifications may be made to adapt a particular situation or object to the teachings of the invention without departing from the essential scope thereof. Accordingly, the invention is not limited to the specific embodiments disclosed, but includes all aspects encompassed within the scope of the appended claims.

Claims (11)

A conduit having an inner peripheral surface and an outer peripheral surface;
A coil adjacent to the outer peripheral surface of the conduit;
A current source connected to the coil and generating an alternating magnetic field at a coil driving frequency;
A plurality of electrodes extending from the outer peripheral surface of the conduit to the inner peripheral surface of the conduit;
A processor that generates a flow rate output signal according to a detected voltage between the plurality of electrodes,
The processor drives the coil according to a complex impedance detected between the plurality of electrodes or between one or more of the plurality of electrodes and a ground as a complex impedance of a process fluid flowing in the pipeline. An electromagnetic flow meter that adjusts one of a frequency, an electrode voltage sampling period, and an electrode signal phase shift . The electromagnetic flowmeter according to claim 1 , wherein the processor adjusts the coil driving frequency in a direction opposite to the change of the complex impedance. The electromagnetic flow meter according to claim 1 , wherein the processor adjusts the electrode voltage sampling period in a direction opposite to the change of the complex impedance.   The electromagnetic flowmeter according to claim 1, wherein the current supply source is a pulsed DC current supply source.   The electromagnetic flow meter according to claim 1, wherein the current supply source is an alternating current supply source. Generating an alternating magnetic field across the flow of the process fluid;
Detecting a voltage induced between a plurality of detection electrodes by the alternating magnetic field;
Generating a flow rate output signal based on the detected voltage;
Detecting a complex impedance between the plurality of detection electrodes or between one or more of the plurality of detection electrodes and a ground as a complex impedance of the process fluid ;
Changing one of a coil drive frequency, an electrode voltage sampling period, and a phase shift of the electrode signal in accordance with the detected complex impedance. The flow rate measuring method according to claim 6 , wherein the electrode voltage sampling period is adjusted in a direction opposite to the detected change in the complex impedance. The flow rate measuring method according to claim 6 , wherein the coil driving frequency is adjusted in a direction opposite to the detected change in the complex impedance. An operation method of an electromagnetic flow meter,
Detecting the complex impedance of the process fluid between a plurality of detection electrodes or between one or more of the plurality of detection electrodes and ground;
Automatically adjusting one of a coil drive frequency, an electrode voltage sampling period, and an electrode signal phase shift of the electromagnetic flow meter according to the detected complex impedance. How to operate. The method according to claim 9 , wherein the electrode voltage sampling period is adjusted in a direction opposite to the detected change in the complex impedance. The method according to claim 9 , wherein the coil driving frequency is adjusted in a direction opposite to the detected change in the complex impedance.

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